Sharpness overconstancy in peripheral vision.

Although much has been learned about the spatial sampling and filtering properties of peripheral vision, little attention has been paid to the remarkably clear appearance of the peripheral visual field. To study the apparent sharpness of stimuli presented in the periphery, we presented Gaussian blurred horizontal edges at 8.3, 16.6, 24, 32, and 40 deg eccentricity. Observers adjusted the sharpness of a similar edge, viewed foveally, to match the appearance of the peripheral stimulus. All observers matched blurred peripheral stimuli with sharper foveal stimuli. We have called this effect "sharpness overconstancy". For field sizes of 4 deg, there was greater overconstancy at larger eccentricities. Scaling the field size of the peripheral stimuli by a cortical magnification factor produced sharpness overconstancy which was independent of eccentricity. In both cases, there was a slight sharpness underconstancy for peripherally presented edges blurred only slightly. We consider various explanations of peripheral sharpness overconstancy.

The effect of spatial frequency and field size on the spread of exclusive visibility in binocular rivalry.

We measured binocular rivalry between dichoptic, orthogonal, sinusoidal gratings both having spatial frequencies of 0.5, 1, 2, 4, 8 or 16 c deg-1 in fields ranging from 0.5 to 8 deg of visual angle in diameter. Total time that one or the other grating was exclusively visible had an inverted U-shaped relationship with spatial frequency, with the peak shifting to coarser spatial frequencies as the field size increased. We computed for each spatial frequency the maximum field size over which a criterion duration of exclusive visibility would spread. When expressed as areas, these sizes were inversely proportional to spatial frequency. This dependence of rivalry on spatial frequency is similar to those for stereopsis and fusion, consistent with the notion that all three binocular phenomena have a common mechanism.

Blur and contrast as pictorial depth cues.

Studies have shown that blur can act as a pictorial cue to depth perception. But blurring a stimulus reduces its contrast, and studies have also shown that contrast can act as a pictorial cue to depth perception. To determine whether blur and contrast have separate influences on depth perception, each variable was independently manipulated in two experiments. Observers reported depth alternations in a simple reversible figure. Both contrast and blur were found to influence depth perception, but blur had its greatest effect at moderate contrasts. When blurred and sharp stimuli were equated on either Michelson or RMS contrast, blur continued to affect depth perception. Hence blur can act as a depth cue independently of contrast. It is speculated that blur is effective as a pictorial cue because of its usual association with other depth cues, particularly in pictures and photographs.